This disclosure relates to arrangements for coupling at least two surfaces, e.g., in terms of a thermal, electric or mechanical connection between the surfaces. Further methods for manufacturing a bridging arrangement or device for coupling two surfaces are presented. The disclosure also relates to devices and methods for filling a gap region between two surfaces, as for example an underfill for flip-chip packages.
In many applications gap regions that are confined by at least two surfaces are filled with particles and cured resins, to provide for a bridging between the surfaces. Often the obtained fillings should have desired physical properties. For example, certain applications may require electrically conducting underfills. Other applications may rely on mechanical rigidity or a specific thermal dissipation through the filling.
In modern electronic devices, for example, substantial gains in performance are achieved by means of circuit miniaturization and by the integration of single-package multi-functional chips. The scalability and performance of such electronic devices are related to their ability to dissipate heat. In typical flip chip arrangements, one integrated circuit (IC) surface is used for heat removal through a heat sink, while the other for power delivery and data communication. Power is delivered throughout solder balls attached to electrical pads on the IC chip that are reflowed and coupled to the main circuit board.
To minimize mechanical stress in the on-chip wiring layers and the solder balls and to protect them electrically, mechanically, and chemically the gap region between, e.g., IC chip and board (created due to the presence of solder balls) is conventionally filled with electrically non-conductive materials, known as underfills. Current efforts towards 3D chip integration, with solder balls as electrical connection between silicon dies, demand high thermally conductive underfills to efficiently dissipate the heat of lower dies to the heat removal embodiment.
Conventional underfills consist of a curable matrix (e.g., epoxy resin) loaded with silica filler particles, which have a similar thermal expansion coefficient (CTE) to that of the silicon. Currently, the requirement of matching CTE dictates the type, and volumetric fill of fillers to be employed in a given underfill. For thermal underfills bridging the surface of an IC chip and a substrate or circuit board the thermal conductivity of filler materials which are used to increase the thermal contact and enhance heat dissipation between connected surfaces should be high. Therefore, Al2O3, AlN, BN or other metal and nonmetal materials are used, for example.
The application of underfills in gap regions is limited by the filler volume fraction, since the resulting viscosity depends on the filler content. According to some conventional methods the underfill material is applied to the chip periphery and capillary forces transports the viscous media into the gap, within a certain time period, prior to a temperature assisted curing. Generally, a high particle load, e.g., >30 vol % is needed to reach thermal conductivity values>0.5 W/m/K. Then the viscosity of the applied medium may become too high to efficiently fill the gaps.